Asparagine slows down the breakdown of storage lipid and degradation of autophagic bodies in sugar-starved embryo axes of germinating lupin seeds

Abstract

The research was conducted on embryo axes of yellow lupin (Lupinus luteus L.), white lupin (Lupinus albus L.) and Andean lupin (Lupinus mutabilis Sweet), which were isolated from imbibed seeds and cultured for 96h in vitro under different conditions of carbon and nitrogen nutrition. Isolated embryo axes were fed with 60mM sucrose or were sugar-starved. The effect of 35mM asparagine (a central amino acid in the metabolism of germinating lupin seeds) and 35mM nitrate (used as an inorganic kind of nitrogen) on growth, storage lipid breakdown and autophagy was investigated. The sugar-starved isolated embryo axes contained more total lipid than axes fed with sucrose, and the content of this storage compound was even higher in sugar-starved isolated embryo axes fed with asparagine. Ultrastructural observations showed that asparagine significantly slowed down decomposition of autophagic bodies, and this allowed detailed analysis of their content. We found peroxisomes inside autophagic bodies in cells of sugar-starved Andean lupin embryo axes fed with asparagine, which led us to conclude that peroxisomes may be degraded during autophagy in sugar-starved isolated lupin embryo axes. One reason for the slower degradation of autophagic bodies was the markedly lower lipolytic activity in axes fed with asparagine.

Regulation of storage lipid metabolism in developing and germinating lupin (Lupinus spp.) seeds

“…Among them were peroxisomes. Such results support another hypothesis that in sugar-starved lupin embryo axes, pexophagy (a special kind of autophagy during which the peroxisomes are selectively degraded) can occur (Borek et al 2013c).…”

Completing Autophagy: Formation and Degradation of the Autophagic Body and Metabolite Salvage in Plants

“…So far in plants, it has only been observed that the degradation of the autophagic body is clearly inhibited by asparagine during sugar starvation-induced autophagy in cells of lupin (Lupinus spp.) embryo axes during seed germination [133]. Enhanced autophagy was found in sugar-starved cells of lupin embryo axes, which was evidenced, among other things, by a high degree of vacuolization and a clear decrease in phosphatidylcholine content [133,138,139].…”

“…embryo axes during seed germination [133]. Enhanced autophagy was found in sugar-starved cells of lupin embryo axes, which was evidenced, among other things, by a high degree of vacuolization and a clear decrease in phosphatidylcholine content [133,138,139]. Under such conditions, the autophagic bodies were rapidly degraded and they were not observed in the enlarged vacuoles.…”

“…In yeast, the main factors that increase the intensity of autophagy are carbon, nitrogen and phosphate starvation [24,129]. In plants, it is known that carbon or nitrogen starvation significantly increases the intensity of autophagy [7,24,29,36,132,133]. In yeast, animal and plant cells, mTOR kinase is the main intracellular center of signal collection associated with autophagy.…”

“…Such accumulation of autophagic bodies inside enlarged vacuoles proves that asparagine only inhibits the degradation of autophagic bodies, but does not reduce the intensity of autophagy in general. The effect of asparagine is similar to the action of concanamycin A (an inhibitor that slows down the breakdown of the autophagic body by lowering the pH of the vacuole); however, the mechanism of the inhibitory action of asparagine is not known [133].…”